Activation of cellulose for acylation



Patented Dec. 6, 1949 ACTIVATION OF CELLULOSE FOR ACYLATION William B.Hincke, Kingsport, Tenn, and George A. Richter, Rochester, N. Y.,assignors to Eastman Kodak Company, Rochester, N. Y., a corporation ofNew Jersey No Drawing. Application August 4, 1945,

Serial No. 609,044

This invention relates to the activation of cellulose sheets in whichthe sheet is dampened or wetted, stored for a time under non-dryingconditions, and then dewatered with a lower fatty acid.

It is well known that cellulose is more readily susceptible to acylationif it is first pretreated or activated. When the cellulose is in aloosely assembled form such as picked cotton linters, treatment for atime with acetic acid has been found to impart sufficient activationthereto, par- 3 Claims. (Cl. 260-212 ticularly when acetic anhydride isemployed for the esterifying agent.

That water has a swelling action on cellulose has been recognized in theprior art. Its use for activating cellulose necessitates a long soakingperiod thus tieing up equipment and in addition anhydride is consumed ifthe moist cellulose is employed directly in the esterification process.

It has been recognized that a convenient form in which cellulose may behandled is in sheet form. The formation of sheets is accomplished bylaying down the pulp slurry on a sheet forming surface and then drying.This drying shrinks the fiber resulting in loss of chemical reactivitydue probably to a hardening of the fiber wall structure. As has beenrecognized in the prior art, cellulose which has been sheeted isgenerally less susceptible to esterification than cellulose which hasnot been so treated. The dry sheet cellulose even after mechanicaldistintegration is very difilcult to react by the ordinary commercialprocesses with lower fatty acid anhydrides so as to result in completeacylation. The residual unreacted material can be removed only withgreat difiiculty from the cellulose ester during the subsequentprocessing steps.

We have found that cellulose in sheet form can be activated while inthat form to a degree that it is readily susceptible to esterification.Our activating method is limited to the use of cellulose sheet which hasbeen dried to a point that unreactive hornified material is presenttherein, such as sheets having a moisture content of less than We havefound that moistening the dry sheet, storing it under humidityconditions such that drying does not occur and then dewatering the sheetwith concentrated lower fatty acid, a cellulose is obtained whichreadily enters in its entirety into an acylation reaction so that noresidual unreacted material remains. In the manufacture of high butyrylcellulose esters (at least butyryl) butyric anhydride is ordinarilyemployed as the esterifying agent. As this anhydride is sluggish in itscellulose esterifying action compared with acetic anhydride, it isparticularly important that the cellulose to be esterified with butyricanhydride be completely reactive.

One object of our invention is to provide a process for activatingcellulose in sheet form in which the convenient sheet form is retainedthroughout the activation. Another object of our invention is to providea process for activating cellulose in which the cellulose when ready foresterification is substantially free of moisture. A further object ofour invention is to provide a method of activating cellulose in sheetform whereby the cellulose is rendered completely susceptible to theusual acylation procedure. Other objects of our invention will appearherein.

Our invention in its broadest aspects comprises applying moisture tocellulose sheet so that a moisture content of about 35-65% (based on thewet sheet) is imparted thereto, storing the sheet under conditions ofsuflicient humidity to prevent drying thereof (such as a relativehumidity of at least for from 1 day to approximately 2 weeks; and thendewatering the sheet with a lower fatty acid of 2-4 carbon atoms,followed by acylation of the so-treated cellulose, such as in anesterification mixture in which butyric anhydride is the esterifyingagent.

An outstanding feature of our invention is the reactivation of cellulosein sheet form. It is ordinarily very convenient to form cellulose into adry sheet for either storing or shipping. For instance as wood pulp ismarketed it is usually in the form of rolls of the dried pulp. In sheetform cellulose may be compacted into a small space. Activation ofcellulose in sheet form avoids the necessity of providing mixers orcontaining vessels in which to carry out the pretreatment. By our methodthe moisture may be directly imparted to the cellulose sheet, which maythen be rolled up or otherwise compacted, in which form it may be storedfor the required time.

The cellulose pretreated by our process may be any acetylation gradecellulose in sheet form. For instance it may be cotton cellulose such aslinters, wood pulp cellulose such as sulfite pulp, kraft pulp or othercellulose material of sufficient viscosity for making cellulose estersof the desired properties. Our invention is also applicable to theactivation of mercerized cellulose, which is even less reactive in dryform than unmercerized cellulose. Our invention is directed to theactivation of dry cellulose sheet, that is having a moisture content ofless than 20%.

The cellulose sheet may be either thin. medium or thick as desired. Forinstance the thickness of the pulp sheet may be from paper thickness(.005") up to /8 inch or even more. The sheet may be composed ofsubstantially unbeaten fiber or in some instances may be composed ofpulp which has undergone a preliminary beating or jordanning. The sheetactivated may be tightly compacted or it may be one in which the fibersare loosely assembled (or bulky).

The water employed is preferably one which has been purified such as byone of the usual water purification treatments, .to. remove any organicmaterial in suspension or inorganic material in solution which may bepresent therein. Distilled water is ideally suited for use in thisconnection. If desired the water may contain a small proportion ofpreservative such as sodium benzoate or acetic acid (approximately 5%)as the presence of these materials will allow longer storage of thecellulose sheet without spoiling. In lieu of the use of preservative,the dry sheet may be sterilized by exposing to ultraviolet light such ason its way to the water applicator roll for this purpose. The water usedshould have no chemicals (except as specified) added thereto,

it: being preferable to employ pure water for activating the cellulose(-20 P. P. M. mineral content).

The water may be applied to the cellulose sheet by any desired method.such as spraying, steamcases warm or hot water may be applied with orwithout a subsequent chilling roll. In the use of spraying it isdesirable to us spore-free air, if air is to be used, to assure theabsence of organisms. After wetting the sheet it is preferred to roll itinto a compactroll and allow it to stand for the time necessary .toimpart .good activation to the cellulose. Thev pH of wet cellulose sheetmay range from -8 depending on the wood pulp used. As the addition. ofchemicals is to be avoided, the pH should be kept atthat naturallyobtained with the pulp used.

' The time neededito obtain maximum. activation depends on severalfactors such. as the temperature, the amount of water present, thehistory of the cellulose fiber and theuse to whichv it is. to be put. Wehave found that under the most favorable conditions activation of thecellulose. may be obtained in as little as 24 hours. These conditionsinclude the use as the starting material of a pulp which has notbeenseverely dried (such as having a. moisture content-of -20%),

conducting the pretreatment at a relatively high.

temperature (such as 180 F.) after imparting a moisture content of50-65% to the cellulose pulp sheet. The range of time within whichactivation may be obtained is from 1 day to 2 weeks. We have foundhoweverthat ordinarily a time within the rangeof 5-10 days issatisfactory for pretreating a pulp' sheet in accordance with ourinvention.

After the pulp sheet has been moistened so that the water content is-65% (based on the wet sheet) it is preferably compacted to save space,facilitate handling, and facilitate the pretreatment operation. If thesheet is continuous it is preferably rolled up into units of suitable.

size such as weighing from 100 to 500 lbs. or more. If the sheet isdiscontinuous the cut sheets may be piled one on the other to thedesired height. The pulp sheet is then stored dehydrated by displacingthe water therein with a lower fatty acid such as acetic, propionic,buiwric or a mixture of any two or all of these acids. This dehydrationis conveniently carried out in the case of the continuous sheet bypassing it, preferably while supported on a continuous perforatedstainless steel belt, over a series of suction boxes and spraying orflooding the sheet with the lower fatty acid between suction boxes. Ifdesired, acid of increasing concentrations may be applied to the sheetas it progresses through the series of suction boxes.

The acid applied to the pulp. to dewater it, serves the additionalfunction of removing coloring matter contained in the pulp. In somecases it may be desirable to apply the acid at a somewhat elevatedtemperature (l-l80 F. for ex ample) to further enhance the removal ofcoloring matter contained in the pulp.

If the wet pulp is in the form of cut sheets, these may be dewatered bypassing lower fatty acid through a column in which these sheets arestacked until substantially complete removal of the water is obtained.In any procedure for the removal of the Water, acetic acid, butyric acidor a mixture of acetic and butyric acids may be employed.

After the cellulose has been activated and a cellulose pulp sheetcontaining lower fatty acid results, it may be sent directly to theesterification. vessel either as a continuous sheet or after. it haspassed through a mechanical shredder. If it is to be butyrylated so asto impart butyryl content thereto, it is usually desirable that theamount of acetic acid present be kept to a minimum. Our invention isparticularly directed to the preparation of butyric acid esters ofcellulose having a high butyryl content such as cellulose acetatebutyrates or cellulose butyrates having a butyryl content greater than25%. However our invention relates broadly to the activation ofcellulose and cellulose activated in accordance with our invention alsomay be acetylated or propionated to form any of the following estersusing the corresponding anhydride or anhydrides: cellulose acetate,cellulose propionate, cellulose acetate propionate.

The following examples illustrate our invention:

Example I One part of rolled sheet wood pulp of acetylation grade havingthe following composition:

or cellulose per cent 96.

p cellulose do 2.2

Gamma cellulose do 1.8 Solubility in 10% KOH do 6.1

Cuprammonium viscosity poises 7-20 uous countercurrent waterdisplacement appara- The water was removed over a series of 8 tus.suction boxes by butyric acid of increasing concentrations. A total of il -5 parts of 100% butyric acid (based on the dry weight of the sheet)was sprayed or flowed onto the sheet on the last stage of the suctionbox. About 1-2 parts of acid and from 0.01 to 0.04 part of water remainin the dewatered cellulose sheet. The other part of the acid is causedto-pass countercurrentwise from suction box to box and alternatelythrough Parts Cellulose (dry weight) 1 Butyric acid 2 Acetic anhydride0.76 Butyric anhydride 2.8 Sulfuric acid catalyst 0.03

This mass reacts rapidly in 4 to 6 hours, the temperature being socontrolled that it does not exceed 100 F., the reaction being carriedout in a sigma bladed jacketed mixer, in the jacket of which coolingwater circulates to control the temperature. There was obtained by thisprocess a very clear, haze-free, smooth, acid dope of cellulose acetateb-utyrate having a butyryl content of 38%. The resulting celluloseacetate butyrate was of fairly high viscosity and was particularlysuitable for use in the manufacture of plastics.

Example II One part of rolled sheeted cotton linters of acetylationgrade having the following composition was used.

Alpha cellulose per cent 99.0-99.5

Solubility in KOH solution do 1.5-2.5 Cuprammonium viscosity (A. C. S.

method) -seconds- 200-500 This rolled sheet was dampened with 40% water(based on the weight of the wet cellulose) by passing it over a waterapplicator and rerolling. The rolled sheet was then let stand for sevendays in a high humidity room at 100 to 130 F. The damp pulp sheet wasthen dewatered by spraying it countercurrentwise with acetic acid in thesame manner as described in Example I.

The loosely picked apart pulp, wet with two parts acetic acid, wascharged to an acetylation reactor to which cold butyric anhydride hadpreviously been charged. The reaction mass, having an initialtemperature of about F., was of the following composition:

Parts Cellulose (dry weight) 1 Acetic acid 2 Butyric anhydride 4Sulfuric acid catalyst .03

This mass reacted rapidly with increasing temperature but thetemperature was controlled so as not to exceed 190 F. A sigma bladedjacketed mixer was used through the jacket of which cooling water wascirculated to control the temperature. After from 4 to 6 hours, a smoothclear acid dope was obtained of a 26% butyryl acetate butyrate mixedester particularly suitable for the manufacture of plastics.

Example III One hundred parts of rolled sheeted cotton linters of thesame acetylaton grade as shown in Example II was dampened with 60 partsof water, re-rolled, and let stand five days in a room at 120-130 F. and-90% relative humidity. The so activated cellulose was then dewatered asdescribed in Example I but using propionic acid instead of butyric.

The dewatered sheet, wet with two parts of propionic acid, was thenpicked apart and charged to a jacketed acylation mixer to which coldpropionic anhydride had previously been added. The reaction mass, havingan initial temperature of about 50 F., was of the following compositionParts Cellulose (dry weight) 1 Propionic acid 2 Propionic anhydride 3.4Sulfuric acid catalyst 0.03

' After initiating the reaction, this mass acylated rapidly andsmoothly. The temperature, controlled by jacket cooling, was notpermitted to raise over F. In 4 to 6 hours a smooth completely reactedacid dope of a tripropionate cellulose ester resulted. The esterprepared was of high vscosity and of good clarity.

Example IV One part of a wood pulp rolled sheet of the same analysis asshown in Example I was activated by wetting and letting stand. After 5-8days it was then dewatered as described in Example I. This pulp sheet,wet with two parts of butyric acid, was then picked apart and charged toa sigma bladed acylation mixer to which four parts of cold butyricanhydride had been added. The reaction mass at an initial temperature of50-55 F. was of the following composition:

Parts Cellulose (dry weight basis) 1 Butyric acid 2 Butyric anhydride 4Sulfuric acid catalyst 0.03

After initiating the reaction, the mass reacted rapidly to a smoothclear acid dope. At a temperature increasing as reaction ensued but notexceeding 80-90 F., the reaction became complete in from 4-6 hours to ahigh viscosity acid 1 dope of cellulose tributyrate.

Example V Cellulose (dry Weight basis) 1 Propionic acid 2 Butyric acid 4Sulfuric acid 0.03

The reaction, initiated at about 50 F.,

75 gradually accelerated as the temperature was al- 7- lowed; to raise;Apeaktemperature of over- 90 F. was avoided. After 4 to 8 hours thereaction was complete to a clear thick acid dope of apropionate-butyrate mixed cellulose ester of a high viscosity.

The cellulose esters of high butyryl and propionyl content prepared fromcellulose activated in accordance with our invention are, eminentlysuitable for the manufacture of molding compositions, melt-coatingcompositions or the like. by mixing with suitable plasticizers. Ifthought desirable, these esters. are readily susceptible to; treatmentby the, method described and claimed in Malm and Crane Patent No.2,346,498, granted April 11-, 1944.

We claim:

1'. A process for pretreating dry fibrous cellu-- lose sheet which hasav moisture content of less than and is resistant to the action ofesterifying, reagents, which comprises moistening the sheet to a pointwhere the, moisture content of the whole is 65%, placing the sheet in anatmosphere having a relative humidity-of at least 60% and a temperatureof 100-180 F. for one day to two weeks whereby the cellulose isactivated, and then dewatering the sheet with fatty acid of 2-4 carbonatoms so as to give a cellulose containing fatty acid but substantiallyfree of Water and readily susceptible to the action of acylatingreagents.

2. A process for, pretreating dry fibrous cellulose sheet which has amoisture content of less than 20% and is resistant to the action ofesterifying, reagents, which comprises moistening the sheet to a pointwhere the, moisture content of the whole is- 30-65%, placing" the sheetin, an atmosphere having a. relative, humidity of; at least and a;temperature of 100-180-F. forone day; to two; weeks whereby thecellulose is activated, and then dewatering, the sheet with acetic acidso as to give a, cellulose containing aceticacid' but substantially freeof water and, readily suseptible to the action of acylating re,- agents.

3. A process for pretreating; dry fibrous cellulose; sheet. having amoisture content: of 0-20%, which comprises moistening the sheet-to a,point where the moisture content of the whole is 50-65%, placingthesheet in an atmosphere-having a: relative humidity of -100% and a,tem -l perature of -180 F. for one day to two weeks: whereby:thezcelluloseis activated and then dewatering the sheetwith acetic acidso as to give a; cellulose containing acetic acidbut substantially freeof water and readily susceptible to theaction of acylating reagents.

WILLIAM B. HINCKE. GEORGE A. RICHTER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,752,596 Hubert Apr; 1, 19302,112,115 Richter Mar. 22, 1938 2,315,973. Malm Apr. 6, 1943 2,379,310Malm etal June 26, 1945 2,380,706 Richter July 31, 19.45

